This invention relates to a frequency-selective surface structure for filtering electromagnetic waves.
For such frequency-selective surface structures which have a fully reflective surface penetrated by slot elements in a repetitive pattern, slot configurations in numerous embodiments are known from the state of the art, starting with simple longitudinal or transverse slots (U.S. Pat. No. 4,314,255), continuing with Jerusalem cross slots and H-shaped slot elements (German Patent Document DE-OS 3 726 309) and ranging to geometrically complex tripolar or multipolar slot elements (U.S. Pat. Nos. 3,975,738 and 4,126,866). However, none of the known slot elements meets the requirement for qualitatively high-value band pass characteristics to an unlimited extent. Either the resulting surface structure has a filter action that is too broad-banded or has excessive transmission losses and/or the resonance frequency is highly dependent on the wave angle or on the polarization.
It is an object of the present invention to provide a frequency-selective surface structure provided with a slot pattern in the case of which a narrow-band filter action with low transmission losses can be achieved that is largely independent of the wave angle and of the polarization.
This object is achieved according to the invention by means of the special geometrical design of an H-shaped slot structure to keep the mutual distance from center to center of the slot elements (and thus the transmission losses and the dependence on the wave angle) very small, without resulting in interfering coupling effects between the individual slot elements, which adversely affect the narrow-band resonance behavior of the surface structure. In addition, the slot elements according to the invention, because of their orthogonal-symmetrical surface geometry which has essentially the same length in the X-direction and in the Y-direction, which for the purpose of polarization-independent band-pass characteristics, can easily be arranged in a slot pattern that is identical in the H-plane and in the E-plane.
In a first preferred embodiment, the polarization-independent filter action is achieved by arranging adjacent slot elements at right angles with respect to one another in each case.
In order to further improve the degree of transmission and stability with respect to wave angle changes by a slot density of the surface structure that is as high as possible without any adversely affecting on the filter band width, the slot elements in a second preferred embodiment, are each positioned with a center distance in both the horizontal and vertical directions of approximately a third of a wave length of the wave frequency to be selected.
The slot width of the individual slot elements is selected according to the desired filter band width and, with a view to a narrow-band filter action, preferably amounts to approximately 1% of the operating wave length in the area of the connecting bars, while the limiting bars are approximately half as wide as the connecting bars.
Another embodiment of the invention, which is particularly preferred because of a high edge steepness of the filter, consists of the fact that the surface structure is constructed as a sandwich component, with two outer cover layers, each penetrated by H-shaped slot elements and an intermediate layer arranged between them which is constructed of a low-loss dielectric, having an effective electrical layer thickness which corresponds to one fourth of the operating wave length. With a view to a favorable transmission action, it was found to be expedient to arrange the H-shaped slot elements in alignment with one another in the two outer cover layers.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.